1. Field of the Invention
The present invention relates to a pneumatic booster for use, for example, in a brake system of a vehicle, for example, an automobile.
2. Description of the Prior Art
These days, many automobiles are equipped with a pneumatic booster which is designed to boost brake pedal pressure by utilizing a pressure difference between a suction pressure produced in an engine and an atmospheric pressure, thereby increasing a braking hydraulic pressure generated in a master cylinder, and thus making it possible to obtain a great braking force with a relatively slight brake pedal pressure.
One example of such a pneumatic booster will be explained with reference to FIG. 1. The pneumatic booster 1, which is of the tandem type, has a housing 4 that comprises a front shell 2 and a rear shell 3. The interior of the housing 4 is divided by a center shell 5 into a pair of front and rear chambers 6 and 7. The front chamber 6 is divided by a front power piston 9 with a front diaphragm 8 into a front constant-pressure chamber 10 and a front variable-pressure chamber 11. Similarly, the rear chamber 7 is divided by a rear power piston 13 with a rear diaphragm 12 into a rear constant-pressure chamber 14 and a rear variable-pressure chamber 15. The constant-pressure chambers 10 and 14 are communicated with a negative pressure source (not shown) so as to be placed under a negative pressure.
The power pistons 9 and 13 are secured to a valve body 16, to which is connected an output rod 17. In addition, a plunger 19 that is connected to an input rod 18 is slidably inserted into the valve body 16. The valve body 16 incorporates a breather valve 20 and a vacuum valve 21. The breather valve 20 allows the variable-pressure chambers 11 and 15 to communicate with the atmosphere when the input rod 18 moves forwardly relative to the valve body 16, whereas the vacuum valve 21 allows the variable-pressure chambers 11 and 15 to communicate with the constant-pressure chambers 10 and 14 when the input rod 18 moves rearwardly relative to the valve body 16.
With this arrangement, when the brake pedal is stepped on, the input rod 18 moves forwardly to open the breather valve 20, so that atmospheric air is introduced into the front and rear variable-pressure chambers 15 and 11 to produce a differential pressure between each of the constant-pressure chambers 10 and 14 and the corresponding one of the variable-pressure chambers 11 and 15, causing the power pistons 9 and 13 to move forwardly. In consequence, the valve body 16 is moved forwardly with great force, thereby transmitting a boosted thrust force to the output rod 17.
Incidentally, with increased engine output being demanded, engines are increasing in size thus imposing a great restriction on available space inside an engine compartment. For this reason, parts in an engine compartment are desired to be reduced in size which requirement equally applies to a pneumatic booster without any lowering in performance.
In the above-described pneumatic booster 1, however, any reduction in the radial dimension (i.e., the outer diameter) has a great effect on the pressure-receiving area of each of the power pistons 9 and 13, which may lead to a lowering in the performance. For this reason, the radial dimension of the pneumatic booster cannot be reduced. It is therefore demanded to reduce the axial length of the pneumatic booster.